Referred to by the acronym V/F, Voice Frequency Telegraphy was used for teleprinter communications during World War Two. The system exploited the wide frequency range of a voice telephone line to transmit a number of different teleprinter signals, each signal being channelled within one part of the available frequency range. In this way, up to 48 different channels could be used to communicate simultaneously on one line. In the 1940s this remained a relatively new technology, and Bletchley had its own V/F technical team from the GPO housed in the Teleprinter Building.

I recall reading that, in the 1870's, Westinghouse were developing a method of sending two or more Morse code signals down a single telegraph line. I believe they were using electro-mechanical reeds operating at different audio frequencies. The reeds could be used for both generating and detecting signals. While developing the technique they found they could hear voices over their system. They drafted a patent and submitted it to the patent office but Alexander Graham Bell had submitted his telephone patent just a few hours before !!